You can detect a circular feature in an image by correlating the image with a kernel image of a circle with the same radius.
I had a quick look at your image (http://img850.imageshack.us/img850/6925/samplerd.png), and found that the following code nicely detects your circular feature.
im = <your image>
im2 = im // ImageAdjust // ColorNegate
ker = {FaceForm[White], EdgeForm[{Black, Thickness[0.1]}], Disk[]} //
Graphics // Rasterize[#, ImageSize -> 140] & // ColorNegate
corr = ImageCorrelate[im2, ker] // ImageAdjust
This solution needs to be tidied up and extended, but this approach gives you a fairly direct way of extracting the centroid and radius as you requested.
Stephen Luttrell